Consumer markets continue to request smaller, portable electronic devices which having more functional features. Examples of such devices include two-way and broadcast radio receivers, compact disc players, cellular telephones, and computer devices to name but a few. As portable electronic devices have become smaller, the demand for smaller energy sources, such as batteries, to power such devices has increased. Obviously very small energy storage devices, such as an electrochemical battery cell, may be fabricated for a given electrical device; however, compactness comes at the cost of energy capacity. Accordingly, for many high power applications the energy source is too bulky, too heavy, or doesn't last long enough.
As the energy storage device, such as a battery, is discharged, it becomes unable to provide current at a required level. Thus, even though the battery may retain a substantial charge, it is useless to the device to which it is attached. This problem is exacerbated when the device to which the battery is attached requires high power (i.e., current pulses) in an operating cycle which otherwise requires a much lower operating current. Such is the case with portable communications devices, such as digital two-way radios and cellular phones when in the transmit mode. These power pulses or spikes require significantly higher current outputs than when the device is receiving or in standby mode.
As the physical size of batteries decreases (to meet size requirements of product designers), the capacity of the battery is reduced. This results in device users needing many batteries if they anticipate being away from a battery charging device for extended periods of time. Alternatively, users may carry portable, high speed, charging devices with them. This however is unacceptable, due to the additional weight associated with the charging device.
Prior art attempts to address the high power spikes entailed providing electrolytic capacitors in the application device. This had the disadvantage of increasing substantially the size of the application device, as electrolytic capacitors are typically very large, cylindrical devices. Other attempts are described in U.S. Pat. No. 5,439,756 to Anani, et al, in which an electrical energy storage device is provided. The device disclosed in the '756 patent includes a battery electrode, a capacitor electrode, and a third electrode as the counter electrode for both the battery and the capacitor electrodes. The device also includes electronics to switch the third electrode between the battery electrode and the capacitor electrode.
Accordingly, what is needed is an energy source which is capable of providing sufficient voltage for the high power pulses required of certain devices, while extending the usable life of the energy source. Such a device should be relatively small, and capable of being easily sized and shaped for a given application.